The present invention relates to inorganic pigment pellets for coloring building materials, lacquers and plastics and to a process for the production thereof and to the use thereof inter alia for the production of pastes, suspensions and dispersion paints.
In order to achieve the optimum perceived color, pigments must be ground into primary particles during processing. The resultant powders produce large quantities of dust and, due to their fineness, have a tendency to stick and adhere in metering apparatus. In the case of toxicologically hazardous substances, measures must thus be taken during processing to counter any risks to man and the environment caused by the resultant dusts. Also, there is increasing market demand for the avoidance of dust nuisances even for harmless inert substances, such as for example iron oxide pigments or titanium dioxide pigments.
The avoidance of dust and improved metering due to good flow properties in order to achieve a consistently good perceived color on use in organic media are objectives pursued when handling pigments. These objectives are achieved to a greater or lesser degree by applying pelletizing processes to pigments. The processes used are, for example, agglomeration or spray drying. Conpaction processes are generally less suitable due to the. limited dispersibility of the resultant pellets.
Essentially two opposing requirements are placed upon pigment pellets: mechanical stability of the pellet simultaneously combined with good dispersion characteristics in the medium used. Mechanical stability is responsible for good transport properties, both during transport between the manufacturer and user and for good metering and flow characteristics during use of the pigments. Mechanical stability is brought about by elevated adhesive forces and is dependent, for example, inter alia upon the quantity of binder or also the compressive pressure applied during shaping. On the other hand, dispersibility is influenced by good grinding before pelletization (wet and dry grinding), by the mechanical energy applied during incorporation (shear forces) and by dispersion auxiliary substances, which reduce the adhesive forces in the dry pellets on incorporation into a medium. However, the use of relatively large quantities of dispersion auxiliary substances is limited by the relative costs of the auxiliary and pigment. Moreover, an elevated content of auxiliary substances correspondingly reduces the color intensity or scattering power. Since variations in color strength are generally less than .+-.5%, the use of additives is limited even if they simultaneously act as coupling agents and dispersion auxiliary substances. Additives must furthermore not impair the service characteristics of the media to be colored, for example strength or notched impact strength in plastics, elastic properties and toughness in elastomers (polymers) and rheological characteristics in paints and lacquers.
Known production processes for pigment pellets are, for example, spray granulation (spray drying using a disk or jet) and agglomeration pelletization (mixers, fluidised bed pelletizers, plates or drums) or compaction processes.
EP-A 0 257 423 U.S. Pat. No. (4,810,305)and DE-A 38 41 848 (U.S. Pat. No. 5,035,748) thus describe spray pelletization using polyorganosiloxanes as hydrophobic, lipophilic additives. The stated atomizing dryer generally results in small particle sizes with an elevated proportion of fines. This means that a substantial proportion of the material is not obtained from the dryer as directly usable pellets, but as fines, which must first be retained in a filter and then returned to the process. In spray dried products, hydrophobing post-treatment results in pellets which have very good flow properties but produce exceptionally large quantities of dust.
EP-A 0 424 896 discloses the production of low-dusting fine pellets in a one-stage process in known intensive mixers. This process uses a low content of waxes in combination with emulsifiers and wetting agents by application of an aqueous dispersion Water contents of 20 to above 50% are generally obtained in this process. These pellets must be dried and separated from oversized and undersized fractions.
DE-A 31 32 303 describes low-dusting, flowable inorganic pigment pellets which are mixed with binders which liquefy under the action of heat (40 to 60.degree. C.) and are pelletized by a screening process using a screening aid (pressure).
EP-A 0 144 940 discloses low-dusting pigment pellets which are produced starting from a filter sludge, by mixing with water, which is at a temperature of approx. 50.degree. C., and adding 0.5 to 10% of surface-active substances and additionally mineral oil or liquefying waxes at 50 to 200.degree. C. until the smearing point is reached. This operation is performed in intensive mixers and is optionally followed by subsequent pelletizing and drying operations. Water is present in the finished product in a quantity of 10 to 15%, which is disadvantageous for incorporation into plastics.
DE-A 28 44 710 (U.S. Pat. No. 4,264,552) describes the pelletization of pigments in a fluidised bed with pelletizing auxiliary substances, wherein dry pigment powder is sprayed with water.
DE-C 28 19 004 (U.S. Pat. No. 4,356,162) describes the pelletization of alkali metal aluminates for the production of cements with the assistance of polyhydroxy compounds.
GB-A 2 158 084 (U.S. Pat. No. 4,624,678) describes the production of dye pellets with solid polyethylene glycols in a melting range of 40 to 150.degree. C., the pellets having a wax content of 33 to 66%, relative to the pigment.
U.S. Pat. No. 4,952,617 describes improved pigment dispersions in aqueous systems using trimethylolpropane or trimethylol ethers. The improvement consists in an improved viscosity of the paste, which results in an extended shelf life.
DE-C 43 36 612 (U.S. Pat. No. 5,634,970) discloses the production of hydrophobized pigment pellets using oils of a specific viscosity in accordance with a modified compaction process.
U.S. Pat. No. 3,843,380 demonstrates the production of pigment pellets using thickeners or thickeners in combination with dispersants.